52 COPE 



Class IV.— MONOCONDYLIA. 



There are two subclasses of Monocondylia : 



Anterior limbs ambulatory, with numerous carpal 

 and metacarpal bones ; two aorta roots ; in- 

 tegument consisting partly of scales ; Reptilia^ 



Anterior limbs volant, with the carpals and meta- 

 carpals more or less coossified and reduced in 

 numbers ; integument consisting in part of 

 feathers ; one aorta root ; Aves.. 



Approximations between these subclasses exist at various 

 points. Thus the Dinosaurian reptiles resemble birds in 

 the structure of the posterior limbs and pelvis ; while 

 among birds the Saururse approach reptiles in the structure 

 of the manus and of the caudal vertebrae. The definitions 

 above given are, however, not violated. 



Subclass I.— REPTILIA. 



The diversities in the orders of reptiles are seen chiefly 

 in the constitution of the posterior parts of the skull ; the 

 shoulder-girdle presents a good many varieties, and the 

 limbs and vertebrae exhibit others. 



In order to understand the homologies of the elements 

 which make up the posterior region of the skull of reptiles 

 generally, it is necessary to become acquainted with tiie skull 

 of the primitive order of the Cotylosauria, and the most nearly 

 allied subclass of the Batrachia, the Stegocephali. These 

 are represented in figures. In these types the temporal fis- 

 sure is crowned with an osseous roof which protects the 

 temporal muscles. This roof contains the elements of the 

 arches which extend from the orbit posteriorly to the sus- 

 pensorium of the quadrate bone. These elements are six 

 in number, and are named as follows, commencing with 

 those that lie next to the parietal and the frontal bones,. 



